Light sensitive recording materials may suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such degradation may occur when a fraction of the imaging light which strikes the photosensitive layer is not absorbed but passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element may cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. It is this effect which leads to image degradation. Silver halide based photographic materials (including photothermographic materials) are prone to this form of image degradation since the photosensitive layers contain light scattering particles. The effect of light scatter on image quality is well documented and is described, for example, in T. H. James "The Theory of the Photographic Process", 4th Edition, Chapter 20, Macmillan 1977.
It is common practice to minimise the effects of light scatter by including a light absorbing layer within the photographic element. To be effective the absorption of this layer must be at the same wavelengths as the sensitivity of the photosensitive layer. In the case of imaging materials coated on transparent base, a light absorbing layer is frequently coated on the reverse side of the base from the photosensitive layer. Such a coating, known as an "antihalation layer", effectively prevents reflection of any light which has passed through the photosensitive layer.
A similar effect may be achieved by a light absorbing layer interposed between the photosensitive layer and the base. This construction, described as an "antihalation underlayer" is applicable to photosensitive coatings on transparent or non-transparent bases. A light absorbing substance may be incorporated into the photosensitive layer itself, in order to absorb scattered light. Substances used for this purpose are known as "acutance dyes". It is also possible to improve image quality by coating a light absorbing layer above the photosensitive layer of a wet processed photographic element. Coatings of this kind, described in U.S. Patent Specification No. 4,312,941 prevent multiple reflections of scattered light between the internal surfaces of a photographic element.
Many substances are known which absorb visible and/or ultraviolet light, that are suitable for image improvement purposes in conventional photographic elements sensitised to wavelengths below 700 nm. Triarylmethane and oxonol dyes in particular, are used extensively in this connection. There is, however, a need for antihalation and acutance dyes which absorb in the near infrared region of the spectrum. Dyes of this type are required for recording materials which are spectrally sensitised to near infrared wavelengths, for example, materials which are designed to record the output of near infrared lasers. Coatings of infrared absorbing pigments such as carbon black may be used, however, the use of this material is objectionable since it does not decolourise during processing and must therefore be coated in a binder which dissolves in developing solution allowing the carbon black to wash off causing objectionable contamination of the developer.
The classes of organic dyes which are commonly employed for antihalation and/or acutance purposes in ultraviolet and visible light sensitive materials do not readily form stable derivatives which absorb strongly in the near infrared. References to visible light herein refer to wavelengths between 400 and 700 nm and references to near infrared light refer to wavelengths between 700 and 1400 nm.
Coatings of antihalation or acutance dyes which absorb in the visible region of the spectrum are usually required to become colourless during processing of the photographic material, either by washing out or chemical reaction in wet processing techniques or thermal bleaching during heat processing techniques. Cyanine dyes are well known in the photographic art and are extensively used as spectral sensitisers for silver halide based materials. Cyanine dyes are not usually used or considered suitable for antihalation or acutance purposes owing to their limited solubility in aqueous media and especially to their inability to decolourise completely during photographic processing when present in large quantities.
Since the human eye is insensitive to near infrared radiation, coatings of dyes which absorb only at wavelengths longer than 700 nm appear colourless and would therefore be acceptable in photographic materials without any change in absorption during processing. Dyes of this type would be suitable as antihalation and acutance dyes in infrared sensitive photographic elements. Heptamethine and longer chain cyanine dyes are known which have absorption maxima in the near infrared region of the spectrum. However, simple near infrared absorbing cyanine dyes exhibit an absorption curve which is broadened on the short wavelength side and extends well into the visible region of the spectrum (see, for example, A. Weissberger and E. C. Taylor, Special Topics in Heterocyclic Chemistry, John Wiley and Sons, 1977, page 504). This extended absorption curve into the visible results in an objectionably high blue or green appearance which does not disappear during photographic processing. It is an object of the invention to provide a class of dyes which are capable of forming coatings which absorb in the near infrared and have a relatively low visible absorption, whilst retaining a very high extinction coefficient at the absorption maximum.